Gytc is still very much a work in progress (and one which I haven't had much time to devote to recently), but my main theory behind keeping the -s is that PGmc -(a)z was rhotacised and/or then disappeared in the other Germanic languages, but Gothic devoiced the -z before it could be rhotacised. I don't think the IE -s ending is terribly anachronistic: It still shows up in plenty of languages like Greek; and in germanic it's still retained in Icelandic and Faroese as -r (again, rhotacised).